1. Field of the Invention:
The present invention relates to an optically driven semiconductor device in which switching operations are performed according to an optical signal. The present invention also relates to a semiconductor relay device, and more particularly it relates to an optically coupled semiconductor relay device, which comprises a light emitting part for generating the optical signal and a photodetector part for performing switching operations according to this optical signal, wherein the above-mentioned optically driven semiconductor device is used as the photodetector part.
2. Description of the Prior Art:
In recent years, with the achievement of rapid developments in the semiconductor technology, there have been increased demands for higher performances and miniaturization of hardware in various control systems, along with the digitalization thereof. However, the input/output parts of these control systems still process analog signals, and when viewed from the whole system, digital circuits and analog circuits are arranged in a mixed form within the same system. Accordingly, it is an important subject that analog signals (particularly, minute voltages and currents) are reliably processed and that the processed signals are introduced into the control part of the system.
As a device for controlling an output-side circuit insulated electrically from an input-side circuit according to the signals given to the input-side circuit, electromagnetic relay devices have been mainly used. However, electromagnetic relay devices have movable mechanical parts, therefore the relay device itself is large in size, and it is difficult to make the equipment using this relay device smaller in size. Moreover, electromagnetic relay devices have disadvantages in that the movable mechanical parts are easily fatigued and the life thereof is short.
In recent years, in place of such electromagnetic relay devices, semiconductor relay devices, referred to as solid state relays (SSRs), having the advantages of small size, light weight, and long life have been widely applied. However, for example, the semiconductor relay devices using bipolar transistors or thyristors can only be used either with alternating current or direct current.
To solve these problems, a semiconductor relay device capable of being used with either alternating current or direct current by the use of two metal oxide semiconductor field effect transistors (MOSFETs) has been developed. This is an optically coupled semiconductor relay device which is composed of a light emitting part, comprising a light emitting diode, and a photodetector part, comprising photodiodes and MOSFETs. FIG. 7 shows an equivalent circuit of the semiconductor relay device. Photodiode arrays 41 and 42 of the photodetector part receive an optical signal emitted from a light emitting diode 38 of the light emitting part and perform photoelectric conversions, so that MOSFETs 39 and 40 are driven by the obtained electromotive forces. The respective source electrodes of the MOSFETs 39 and 40 are connected to each other, and the respective electromotive forces of the photodiode arrays 41 and 42 are applied between the gate electrode and the source electrode of each of the MOSFETs 39 and 40, respectively. Thus, the current flowing between the drains of the MOSFETs 39 and 40 is controlled.
The above-mentioned optically coupled semiconductor relay device requires a total of five elements (i.e., one light emitting diode, two photodiode arrays, and two MOSFETs). As a method for miniaturizing the semiconductor relay device and for reducing the manufacturing cost thereof by further decreasing the number of elements to be used, a method has been proposed in which the photodiode arrays are disposed on an insulating film above the vertical MOSFETs. By adopting this method, the MOSFET and the photodiode array are formed integrally, so that the optically coupled semiconductor relay device can be realized with the use of a total of three elements.
However, in the above-mentioned method, it is required to apply wiring between two vertical MOSFETs with photodiode arrays disposed thereon. Accordingly, the production cost of the relay device is still high and the miniaturization of the relay device is limited.
On the other hand, to solve the above-mentioned problems, photo MOS relays have been put in practical use. The photo MOS relay is a kind of optically coupled semiconductor relay device, in which the input part and the output part are electrically insulated completely from each other, but are coupled optically with each other. When a current flows through the input side of this photo MOS relay, a light emitting element (e.g., a light emitting diode) emits light, and an optoelectric transducer (e.g., a small-sized solar cell of high voltage output type) receives this optical output, thereby generating a photoelectromotive force. This photoelectromotive force is applied to a power MOSFET as a gate voltage, and this power MOSFET is put in on-state or off-state, thereby making it possible to control a load of the output side.
However, in the conventional photo MOS relay, the optoelectric transducer is disposed so as to face the light emitting device in the same package, and the power MOSFET driven by the electromotive force of this optoelectric transducer is further incorporated thereinto. Accordingly, it is required to mount three or more independent chips in the same package, and therefore there exist several problems, in that the structure of the relay device is complicated, the production cost becomes higher, and the reliability of the relay device itself is reduced. In addition, the above-mentioned power MOSFET and optoelectric transducer are produced individually using separate substrates, and therefore there exists some problems in that no portions can be disposed, which are utilized in common by these elements, and no manufacturing processes can be employed which are used in common for these elements, so causing the production cost to rise.
To solve such problems, it has been proposed that the power MOSFET and the optoelectric transducer are disposed on the same substrate. For example, in the Japanese Laid-Open Patent Publication No. SHO-106660/1987, there has been disclosed a semiconductor device in which an optoelectric transducer is disposed on an insulating film above the semiconductor substrate containing MOSFETs and the like formed thereon. The optoelectric transducer is formed with amorphous Si or monocrystalline Si grown on the insulating film.
However, in the case where amorphous Si is used for the optoelectric transducer, when amorphous Si is deposited on the lower electrode of a predetermined pattern formed on the insulating film, for example, by the discharge plasma method, the MOSFETs on the same substrate suffer serious damage, so that the threshold voltage is varied. Although heat treatment at high temperatures are required to recover this varied value, the amorphous Si film is destroyed during this treatment, so that the recovery of the threshold voltage is difficult. Accordingly, there is a possibility that the device characteristics of the MOSFETs disposed on the same substrate deviate from the design values. Moreover, when the amorphous solar cell is formed, electrodes are required to be provided on the upper and lower sides of the amorphous Si film, so that the manufacturing process is complicated, causing the production cost to rise. Furthermore, the optoelectric transducer using amorphous Si has a possibility that the photoelectric conversion characteristics deteriorate with use over a long period of time, causing lack of reliability.
On the other hand, in the case where monocrystalline Si is used for the photoelectric transducer, there has been proposed a method in which a polycrystalline thin film or amorphous thin film of Si is formed on an insulating film, and then the thin film of Si is irradiated with laser beams and melted, after which it is cooled to form monocrystals. However, such a method has the disadvantages in that the manufacturing process is complicated, causing the productivity to become poor and the production yield of the device is low, causing the production cost to rise.